The present invention is related to digital-to-analog converters (DACs).
Current steering DAC is based on an array of matched current cells elements that are steered to the DAC output depending on the digital input code. The DAC can be implemented either in a binary or unary structures. In a binary implementation, a current switched to the output is directly proportional to the binary weight of the corresponding bit in the input word. The binary weighted DAC suffer from large mismatch errors due to a number of current source switches during code change. In contrary, for the unary implementation, every current source provides the same current, and is switched independently. The digital input code is first decoded to thermometer code. This thermometer code is then used to control the switches. The fully unary implementations can obtain good linearity performance but add complexity and spend more power consumption.
The segmented implementation combines the previous two structures by introducing thermometer decoding for a few of the most significant bits (MSB's) and using binary weight structure on the few least significant bits (LSB's) bits. Segmentation DAC provides a trade-off between complexity, speed, power and performance.
Due to the process variation during IC fabrication, the attributes of transistors such as oxide thickness, width and length vary. For analog circuit, this variance can be represented as mismatch. Transistor mismatch in the current sources of DAC introduces different amount of current into the load. This mismatch error is the main source of converter nonlinearity.
In order to minimize the mismatch between different devices, the same device dimension should be used. Use multiple of the same dimension device can achieve a better matching for current sources. However, for high resolution, low power featured segmented current steering DAC, the device is decreased such that the device size is limited by the technology design rule. It is very difficult to get good differential non-linearity (DNL) for high resolution and low power current steering DAC.